This patent application is related to solid state light emitting devices.
Solid-state light sources, such as light emitting diodes (LEDs) and laser diodes, can offer significant advantages over incandescent or fluorescent lighting. Solid-state light sources are generally more efficient and produce less heat than traditional incandescent or fluorescent lights. When LEDs or laser diodes are placed in arrays of red, green and blue elements, they can act as a source for white light or as a multi-colored display. Although solid-state lighting offers certain advantages, conventional semiconductor structures and devices used for solid-state lighting are relatively expensive. The high cost of solid-state light emitting devices is partially related to their relatively complex and time-consuming manufacturing process.
Referring to FIG. 1, a prior art LED structure 100 includes a substrate 105, such as sapphire. A buffer layer 110 is formed on the substrate 105. The buffer layer 110 serves primarily as a wetting layer to promote smooth, uniform coverage of the sapphire substrate. The buffer layer 310 is typically deposited as a thin amorphous layer using Metal Organic Chemical Vapor Deposition (MOCVD). An n-doped Group III-V compound layer 120 is formed on the buffer layer 110. The n-doped Group III-V compound layer 120 is typically made of GaN. An InGaN quantum-well layer 130 is formed on the n-doped Group III-V compound layer 120. An active Group III-V compound layer 140 is then formed on the InGaN quantum-well layer 130. A p-doped Group III-V compound layer 150 is formed on the layer 140. A p-electrode 160 (anode) is formed on the p-doped Group III-V compound layer 150. An n-electrode 170 (cathode) is formed on the n-doped Group III-V compound layer 120.
The GaN crystal has different electric properties along different crystal directions. The (0001) crystal planes are perpendicular to the c-axis and have the highest electric polarity compared to other planes. The (1-100) crystal planes are perpendicular to the m-axis and are non-polar. Other GaN crystal planes such as (1-101) are semi-polar and have electric polarity less than that of the (0001) crystal planes.
Different crystal planes of GaN crystal also have different optical properties. The internal quantum efficiency (IQE) is the highest for the non-polar (1-100) crystal planes and lower for the semi-polar crystal planes, such as (0001) plane. The polar (0001) crystal planes have the lowest quantum efficiency. In a light emitting device, it is desirable to produce light emission from the non-polar or semi-polar crystal planes to obtain high emission intensity.
Early GaN LEDs had been formed on sapphire, silicon carbide, or spinel substrates (105 in FIG. 1). Recently, attempts have been made to grow GaN light emitting devices having non-polar emission surfaces on LiAlO2 substrates. Although it was found that the light emission of these LED structures were spectrally stable and polarized, the emission intensities were low due to numerous defects formed in the GaN crystals during growth on the LiAlO2 substrate.